Fix problems with default-cases in async code.

tests/compiler/dart2js/async_await_js_transform_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import "package:compiler/src/js/js.dart";
 import "package:compiler/src/js/rewrite_async.dart";
 
 import "backend_dart/dart_printer_test.dart" show PrintDiagnosticListener;
 
@@ skipping 401 matching lines @@
               __goto = 7;
               break;
             case 1:
               // goto case
               __goto = 8;
               break;
             case 2:
               // goto case
               __goto = 9;
               break;
+            default:
+              // goto after switch
+              __goto = 5;
+              break;
           }
-          // goto after switch
-          __goto = 5;
           break;
         case 6:
           // case
         case 7:
           // case
           __goto = 10;
           return thenHelper(foo(), __body, __completer);
         case 10:
           // returning from await.
           // goto while condition
@@ skipping 572 matching lines @@
           // return
           return thenHelper(__returnValue, 0, __completer, null);
         case 2:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, __body, __completer, null);
 }""");
 }